For words that only have 3 letters, there is no 4th letter. For example with the word “increase”, there isn't any extra letters Later on and thus needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've employed “addi” for “addict”. Art is https://socialbuzztoday.com/story4313583/considerations-to-know-about-bip39-word-list-pdf